>All of these problems are easier with higher density, not harder. Would you rather run and maintain a few giant sewer lines for a million people, or thousands of small sewer lines over hundreds of miles?

But that's not really the problem faced with construction, except when building on previously undeveloped land. Significant increase in density imposes commentate additional demand on existing infrastructure, which is much harder to expand than to operate and quite probably vastly harder to expand than it would've been to build initially to that expanded state before there was a dense city on top of it.